Fire resistance of reinforced concrete tubings of underground structures with polypropylene fiber

Introduction. Reinforced concrete structures made of heavy concrete of high humidity (more than 3.5 %) are ­prone to explosive destruction. This phenomenon can lead to premature onset of the fire resistance of such structures and the partial or complete collapse of buildings and structures. The increased humidity of reinforced ­concrete structures is usually found in underground structures and newly constructed buildings. The fire resistance of reinforced concrete tubes of underground structures largely depends on the explosive (brittle) destruction of concrete when exposed to high temperatures of fire on the surface of the tunnel lining.
Materials and methods. As the object of the study were selected reinforced concrete tubes of heavy concrete with a moisture content of 6 % with the addition of polypropylene fiber in the amount of 1 kg/m³. Large-scale fire tests were carried out on a specially manufactured test bench when loading samples with vertical and horizontal loads.
Results and discussion. The main results on the study of fire resistance of reinforced concrete tubes with the addition of polypropylene fiber and without additives are presented. According to the results of experimental studies, it was established that the fire resistance limit of reinforced concrete tubing with the addition of polypropylene ­fiber according to GOST 30247.0–94 was at least 125 minutes (REI 120). The analytical model of fire resistance assessment is developed. To solve the thermal engineering problem, a numerical experiment was performed in the ANSYS software package. An analytical dependence is proposed for determining an additional temperature deflection for a geometrically nonlinear element. The calculation of the fire resistance limit of reinforced concrete tubing with the addition of polypropylene fiber according to the developed analytical model, considering the pre­viously obtained strength and thermal characteristics, confirmed the results of fire tests, and amounted to REI 120.
Conclusion. The use of reinforced concrete tubing made of fiber-reinforced concrete with polypropylene fiber for building envelopes will significantly reduce the cost of a fire protection device and shorten the construction time.

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